In Macbeth how many murderers attacked Banquo and his son?

Although Macbeth had arranged the details of the attack on Banquo and Fleance with two murderers, when the men lie in wait for father and son, we find three murderers on the scene. The first two question the third, asking him who sent him, and he responds, "Macbeth."

The audience is left to speculate about the reason for the third murderer. Does Macbeth add another man to improve the odds by having three men attack two? That thought may well be his reason for the additional murderer.

At any rate, the plan does not succeed because only Banquo is killed. Fleance manages to escape in the darkness; thus, Macbeth is unable to thwart the witches' prophecy that Banquo's descendants will be kings.